This is the answer I have.
Cats owe some of their nine lives to their unique skeletal structure. Cats don’t have a collarbone, and the bones in their spine are more flexible than other animals. This makes it easier from them to bend and rotate their bodies easier during a short fall. A fall of two or more floors, however, can seriously injure a cat. A cat's feet and legs usually can’t absorb the impact of a fall from that distance or higher.